Seeing something interesting from a new perspective can get that writing itch going. I want to share the new angle with readers, in the hope they will carry the idea away with them after they finish the story and chew on it for awhile. The fresh view might come from a dream, a walk in nature, a spiritual experience, or a news article, especially in science. It usually deals with 'what if.'
I'm presently working on a fantasy that began after I read that a male and female mosquito will hum in harmony when they are courting. The book doesn't revolve around this; it acted more as a kick-start for ideas.